Alloy wheels are being increasingly used in the automobile industry to equip both cars and small and medium-sized commercial vehicles and they are particularly appreciated because, besides giving the motor vehicle a particularly attractive appearance, they present mechanical characteristics, such as light weight and rigidity, that are decidedly better with respect to wheels made in the traditional way.
An alloy wheel presents an axle and comprises a hub, a rim, which are situated concentrically around the axle and an intermediate portion, which has the function of connecting the hub to the rim and is made in a very high number of models to give each wheel a distinctive character. Generally, the aforementioned models of the intermediate portion can be classified in a first family, according to which the hub and the rim are connected by a plurality of spokes, and in a second family, according to which the hub and the rim are connected by a perforated plate. Moreover, alloy wheels are made both in a single piece, that is the hub, the rim and the intermediate portion are formed of a single piece obtained by casting or by forging, and in a number of pieces, generally two, that is the hub, a part of the rim and the intermediate portion are made in a first piece obtained by casting or forging, while a further part of the rim is made separately, also by casting or forging, in a second piece, which is later assembled with the first piece. The alloy wheel formed of several pieces is usually defined as being of compound type.
In both cases, the realisation of an alloy wheel contemplates a procedure of casting an alloy of aluminium or magnesium to make an untreated wheel or the pieces that make up the wheel, a heat treatment and a first and a second machining with a turning lathe. As an alternative to casting, the wheel is forged and, afterwards, subjected to heat treatment. The machining operations have the function of realising finished surfaces with high degrees of tolerance along the rim to guarantee a perfect coupling with the tyre and at the hub in the coupling area with the end part of an axle or of a semi-axle of a motor vehicle. The machining also has the function of eliminating burrs and of correcting any imprecisions derived from the previous operations. In other words, the untreated wheel presents eccentric masses which must be removed in such a way that the finished wheel, in use, is as balanced as possible in rotation around its own axis so as not to transmit vibrations to the motor vehicle.
Whereas said result was once accepted as satisfactory by the automobile industry, car manufacturers are now beginning to demand decidedly higher levels of balancing in alloy wheels since car manufacturers are, on the one hand, obliged to reduce the lead weights used for balancing wheels for environmental reasons and, on the other hand, to offer ever higher levels of comfort.
According to a method for producing alloy wheels for motor vehicles disclosed in patent application EP 607,757, the alloy wheels are realised and finished with a cutting machine tool. In particular, the above identified method comprises the steps of measuring the unbalance of said wheel, checking whether said unbalance is lower than an unbalance acceptability value by means of a control unit; calculating a mass to be removed and the respective phase with respect to a determined point on the wheel; said unbalance being identified by said mass and by said phase. The identified mass is removed by the cutting machine tool by offsetting the centre axis of the wheel.
Even though, the above method is a step forward in balancing the alloy wheel and allows reducing the lead applied to the outer side rim, it cannot solve completely the problem set forth above. In fact, EP 607,757 the dynamic unbalance is poorly compensate by machining the wheel by offsetting the axis of the wheel. From DE 24,55,279 it is known a method for balancing the wheel with a mounted tyre by deforming the rim of the wheel. This technique is applicable solely to wheel made of malleable material such as deep drawn metal sheet.